Two common types of heating systems used in commercial and some residential buildings include staged heating systems and heating systems with multiple speed or variable speed blower motors. These heating, ventilating and cooling (HVAC) systems are governed by American Society for Heating, Refrigeration & Air conditioning Equipment, “ASHRAE” Standards. ASHRAE Standard 62 specifies the minimum ventilation rates and the minimum indoor air quality that is acceptable to human occupants. This standard is intended to minimize the potential for adverse health effects caused by inadequate ventilation rates which may result in poor indoor air quality. In addition, proposed addenda N of ASHRAE Standard 62 requires an increase in ventilation flow rate if the discharge temperature is too high, such as greater than 90° F.
Minimum ventilation rates governed by ASHRAE, include requirements for a minimum air flow velocity. One of those requirements is that the air flow velocity reach one-half of the way down an external wall, i.e. the air that is forced out of a duct along the ceiling and toward an external wall must travel at least one-half of the way down the external wall.
As noted above, some common heating and ventilating systems use staged heat controllers. Staged heat controllers typically employ a constant speed blower motor and add or subtract heating elements to maintain, increase, or decrease the amount of heat output into an area. Because staged heat controllers use a constant speed blower and do not limit the outlet air temperature, staged heat controllers are prone to problems, such as air stratification. Air stratification occurs when hot air stays along the ceiling or in one area, and is often a result of the discharge temperature being too high and the air velocity being too low. Hot air naturally rises so it takes a higher air flow velocity to force hot air to travel one-half way down the exterior wall than it takes to force cooler air to travel one-half way down the wall. As a result, hot air output from staged heat controllers often stratifies along the ceiling because the outlet air is too hot and the constant speed blower does not generate the necessary air flow velocity to force the air to travel half way down the exterior walls.
Heating and ventilating systems having multiple speed or variable speed ventilator/blower motors are also commonly used. In this type of system the controller typically ramps the speed of the motor up slowly while the heating element is first energized to minimize discomfort generated by forcing an initial blast of cold air into the space being heated. After the heating element is fully energized and is operating at its maximum heat output the motor runs at full speed. When the heating element is turned off, the variable speed motor speed slowly ramps down as the heating element cools. This process increases the efficiency of the heating system by extracting the remaining heat from the heating element. The heat output from these systems generally is not controlled and as a result, problems such as, for example, air stratification occur in these types of systems as well. In addition, the outlet temperature is often greater than a desired temperature, such as, for example, 90° F.
In addition, new construction materials, such as, exterior glass with improved thermal insulation characteristics and improved building insulation have created additional problems with the above controllers. The improved insulating qualities have reduced the demand for heat in new construction. The reduced demand for heat results in the heating and ventilating system needing to run for shorter periods of time to heat the building. Heating and ventilating systems, however, serve multiple purposes including both heating and ventilating. Inadequate ventilation and poor air quality occurs if the ventilation system is not run for long enough periods of time. As a result, staged heat controllers and typical multi-speed controllers do not provide adequate heating and ventilating control.